Although refineries produce many products, the most desirable are the transportation fuels gasolines, diesel fuels, and jet fuels, as well as light heating oils, all of which are high-volume, high value products. While light heating oils are not transportation fuels, their hydrocarbon components are interchangeable with diesel and jet fuels, differing primarily in their additives. Thus, it is a major objective of petroleum refineries to convert as much of the barrel of crude oil into transportation fuels as is economically practical. The quality of crude oils is expected to slowly worsen with increasing levels of sulfur and metals content and higher densities. Greater densities mean that more of the crude oil will boil above about 560.degree. C., and thus will contain higher levels of Conradson Carbon and/or metal components. Historically, this high-boiling material, or residua, has been used as heavy fuel oil, but the demand for these heavy fuel oils has been decreasing because of stricter environmental requirements. This places greater emphasis on refineries to process the entire barrel of crude to more valuable lower boiling products.
The most important and widely used refinery process for converting heavy oils into more valuable gasoline and lighter products is fluid catalytic cracking, "FCC". FCC converts heavy feeds, primarily gas oils, into lighter products by catalytically cracking larger molecules into smaller molecules. FCC catalysts, having a powder consistency, circulate between a cracking reactor and a catalyst regenerator. Hydrocarbon feedstock contacts hot regenerated catalyst in the cracking reactor where it vaporizes and cracks at temperatures from about 420.degree. C. to about 590.degree. C. The cracking reaction causes combustible carbonaceous hydrocarbons, or coke, to deposit on the catalyst particles, thereby resulting in deactivation of the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, typically with steam, in a stripping zone. The stripped catalyst is then sent to a regenerator where it is regenerated by burning coke from the catalyst with an oxygen containing gas, preferably air. During regeneration, the catalyst is heated to relatively high temperatures and is recycled to the reactor where it contacts and cracks fresh feedstock. CO-containing flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Typical fluid catalytic cracking feedstocks are gas oils having a boiling range from about 315.degree. C. to about 560.degree. C. Feedstocks boiling in excess of about 560.degree. C., typically vacuum and atmospheric resids, are usually high in Conradson Carbon residues and metal compounds, such as nickel and vanadium, which are undesirable as FCC feedstocks. There is increasing pressure to use greater amounts of such heavy feeds as an additional feed to FCC units. However, two major factors have opposed this pressure, namely, the Conradson Carbon residues and metal values of the residua. As the Conradson Carbon residues and metal values have increased in feeds charged to FCC units, capacity and efficiency of FCC units have been adversely affected. High Conradson Carbon residues in FCC feedstocks has resulted in an increase in the portion of feedstock converted to "coke" deposits on the surface of FCC catalysts. As coke builds up on the catalyst, the active surface of the catalyst is rendered inactive for the desired activity. This additional coke build-up also presents problems in the regeneration step when coke is burned-off because the burning of additional coke can cause the temperature in the regenerator to increase to levels which will damage the catalyst. Thus, as the Conradson Carbon residues in feedstocks have increased, coke burning capacity has become a bottle-neck, thereby resulting in a reduction in the rate at which feedstocks are charged to the FCC unit. In addition, part of the feedstock would inevitably be diverted to undesirable, less valuable reaction products.
Furthermore, metals, such as nickel and vanadium, in FCC feedstocks have tended to catalyze the production of coke and hydrogen. Such metals have also tended to be deposited and accumulated on the catalyst as the molecules in which they occur are cracked. This has further increased coke production with its accompanying problems. Excessive hydrogen production has also caused a bottle-neck in processing lighter ends of cracked products through fractionation equipment to separate valuable components, primarily propane, butane and olefins of like carbon number. Hydrogen, being incondensible in a "gas plant", has occupied space as a gas in the compression and fractionation train and has tended to overload the system when excessive amounts are produced by high metal content catalysts. This has required a reduction in charge rates to maintain FCC units and their auxiliaries operative.
These problems have long been recognized in the art. Various methods have been proposed to reduce the Conradson Carbon residue, and metal-containing components in feedstocks, such as resids, before they are sent to an FCC process unit. For example, coking is used to convert high Conradson Carbon and metal-containing components of resids to coke and to a vaporized fraction that includes the more valuable lower boiling products. The two types of coking most commonly commercially practiced are delayed coking and fluidized bed coking. In delayed coking, the resid is heated in a furnace and passed to large drums maintained at temperatures from about 415.degree. C. to 450.degree. C. During a long residence time in the drum at such temperatures, the resid is converted to coke. Liquid products are taken off the top for recovery as "coker gasoline", "coker gas oil", and gas. Conventional fluidized bed coking process units typically include a coking reactor and a burner. A petroleum feedstock is introduced into the coking reactor containing a fluidized bed of hot, fine, inert particles (coke), and is distributed uniformly over the surfaces of the particles where it is cracked to vapors and coke. The vapors pass through a cyclone which removes most of the entrained particles. The vapor is then discharged into a scrubbing zone where the remaining coke particles are removed and the products are cooled to condense heavy liquids. A slurry fraction, which usually contains from about 1 to about 3 wt. % coke particles, is recycled to extinction in the coking zone.
While resid can be upgraded in petroleum refineries to meet the criteria as an FCC feed, there is still a substantial need in the art for more efficient and cost effective methods for achieving this upgrading. There is also a need to increase the amount of liquid products and to decrease the amount of gas and/or coke make when upgrading such feedstocks.